In the multi-carrier systems, the data Peak-to-Average power Ratio (PAR) after carrier combination is very high. For example, in TD-SCDMA, all users and carriers in one cell use the same Basic Mid-amble Index (BMI). Due to high BMI correlation of different users and carriers, PAR of mid-amble code is extremely high when the number of carriers increases. High PAR is a great challenge to the multi-carrier systems.
Conventionally, there are two phase shift approaches to reduce PAR: Static Phase Shift (SPS) and Dynamic Phase Shift (DPS). SPS uses a static phase table to adjust the phase offset for every carrier, while DPS uses a dynamic phase table calculated according to real-time data stream to reduce PAR. Generally, the performance of DPS is better than SPS by tracing the characteristics of data stream.
The existing various DPS approaches are implemented in baseband units. Many complicated processing functional blocks for evaluating PAR, such as DUC, CC, PAR and phase searching, are required. They increase the complexity of Software and Hardware implementation.
Further, the existing DPS approaches implemented in baseband can not deal with PAR fluctuation caused by the frequency quantization error in Numerical Controlled Oscillators (NCO) which is implemented in Digital Front End (DFE). NCO is always implemented by a look-up table approach, but there are quantization errors in phase step. Although this phase error is very small, it can be accumulated so as to impact relative phase among different carriers. Even if a best group of phases is found for all carriers, the relative phases will fluctuate periodically. The fluctuation cycle is some seconds or several minutes which is relative to the frequency error value.